This invention relates to the field of surface treatments, more particularly to surface treatments for micromechanical devices, most particularly to systems and processes for the efficient application of surface treatments on micromechanical devices.
Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the DMD surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device.
Due to the very small size of the micromechanical devices, attractive forces between the various components of the micromechanical devicesxe2x80x94most notably van der Waals forcexe2x80x94are capable of overcoming the operational forces of the device. For example, van der Waals force creates an attraction, often referred to as stiction, between the micromirror and the underlying landing electrode that is capable of keeping the mirror permanently deflectedxe2x80x94ruining the micromirror device.
One method of preventing the effects of van der Waals forces uses a surface treatment, or passivation layer to reduce the attraction between components of the micromechanical device. One such surface treatment is perfluorodecanoic acid (PFDA). PFDA is comprised of a COOH terminal group that bonds to the surfaces of the micromechanical devices, a CF3 terminal group that is relatively inert, and a carbon chain linking the two terminal groups. Other surface treatments are also available. For example, similar molecules with either longer or shorter carbon chains also provide suitable surface treatments in some applications.
When applied to the plasma-cleaned metal surface of a micromechanical device, the PFDA forms a self-aligned monolayer of closely-packed molecules. The CF3 terminal groups form the exposed surface of the monolayer leaving a relatively inert surface which prevents stiction between the component parts of the micromechanical device.
While PFDA solves the stiction problem in micromechanical devices, its application presents many difficulties. Existing methods are quite wasteful since virtually all of the PFDA to which the micromechanical device is exposed is exhausted as waste. PFDA is a toxic material and is disposed of as a toxic acid. Furthermore, existing application methods tend to both clog and corrode the application equipment used. Thus, existing methods risk exposure to toxic materials, create excessive hazardous waste, have high labor costs to maintain equipment, and are wasteful of the raw PFDA material. What is needed is a better system and method for the application of the surface treatments.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for the efficient deposition and recapture of a surface treatment material. According to one embodiment of the claimed invention, a method of applying a surface treatment. The method comprising the steps of providing a surface treatment material, typically PFDA, in a source chamber; evacuating a deposition chamber enclosing an object to be treated; heating the source chamber, the deposition chamber, the object, and an upper portion of a recovery chamber to a temperature above the evaporation point of the surface treatment material; heating the surface treatment material in the presence of a carrier gas to evaporate the surface treatment material; exposing the object to the carrier gas and the evaporated surface treatment material; removing the carrier gas and the evaporated surface treatment material to the recovery chamber; and recovering the evaporated surface treatment vapor or material in the recovery chamber. The surface treatment material is typically recovered in a lower portion of the recovery chamber, which is maintained at a temperature below the freezing point of the PFDA vapor.
The evaporated surface treatment material is typically recovered by condensing the evaporated surface treatment in a removable lower portion of said recovery chamber. Multiple deposition chambers are optionally used with the deposition system, and each deposition chamber is independently emptied and refilled with objects to be treated between delivery of the surface treatment-charged carrier gas. After the supply of source material is exhausted, the source chamber may be cleaned by opening a lower portion of the source chamber.
The system is reversed by the additional steps of: cooling a lower portion of the source chamber; heating the lower portion of the recovery chamber to evaporate the recovered source material into the carrier gas; transferring the evaporated recovered source material and the carrier gas into the deposition chamber; removing the carrier gas and the evaporated recovered surface treatment material to the source chamber; and condensing the evaporated recovered surface treatment material in the lower portion of the source chamber.
According to another embodiment, a system for applying a surface treatment to an object is disclosed. The system comprises: a source chamber for holding a source of surface treatment material; a deposition chamber enclosing the object; a recovery chamber; conduit connecting the source chamber to the deposition chamber and the deposition chamber to the recovery chamber and controlling the flow of a carrier gas between the source chamber, the deposition chamber and the recovery chamber; a supply of the carrier gas; and a heater for heating the source chamber, the source of surface treatment material, the deposition chamber, an upper portion of the recovery chamber, the carrier gas, and the conduit.
In operation, the heater evaporates the surface treatment material from the source container into the carrier gas, the conduit transfers the evaporated surface treatment material into the deposition chamber where a portion of the evaporated surface treatment material is deposited on the object, the conduit further transferring a remaining portion of the surface treatment material to the recovery chamber where it condenses in a lower portion of the recovery chamber.